The invention relates to a hydraulic brake system comprising a master brake cylinder, a brake force booster and a filling chamber by means of which in the initial phase of a brake actuation the brake system on relatively short pedal travel is rapidly filled with hydraulic fluid up to a predetermined switch-off pressure, the filling chamber being in conductive communication with a primary chamber of the master brake cylinder and connectable to a hydraulic fluid reservoir via a first valve which closes when the brake force booster is functioning and a pressure obtains in the hydraulic fluid below the switch-off pressure and opens when the brake force booster is not functioning.
Thus, in such a hydraulic brake system known from DE-OS 3,508,753 the usual master brake cylinder is supplemented by a brake force booster likewise known per se and a so-called filling stage. Filling stages are known per se for example from U.S. Pat. No. 4,086,770. A filling stage achieves that on a brake actuation in the initial phase of the pedal travel the brake system is rapidly filled with hydraulic fluid. The driver therefore need only press the brake pedal through a relatively short travel. As soon as a predetermined pressure is reached in the hydraulic fluid, the so-called switch-off pressure, the filling stage switches off, i.e. becomes ineffective. On further pressing down of the brake pedal hardly any hydraulic fluid is then moved in the brake system, the pedal pressure instead being directly converted hydraulically to brake pressure.
Brake force intensifiers or boosters, known for example from DE-OS 2,450,474 and U.S. Pat. No. 3,109,287, assist the action of the brake pedal and as a rule comprise two chambers with a movable partition which is referred to as diaphragm plate. By means of the engine a pressure difference is generated between the chambers so that the diaphragm plate assists the action of the brake pedal actuated by the driver.
In DE-OS 3,508,753, cited at the beginning, the filling stage and the brake force booster are designed to be dependent on each other in their function, i.e. the filling stage and brake force booster do not function independently of each other. This is based on the recognition that on failure of the brake force booster considerable problems can result from the driver then having to apply directly via the brake pedal (in addition to the normal pressure buildup in the master brake cylinder) the actuating force for the filling stage up to the relatively high switch-off pressure. The resulting change in the pedal resistance can irritate the driver.
DE-OS 3,508,753 therefore avoids the driver detecting troublesome irregularities in the resistance of the brake pedal on failure of the brake force booster. For this purpose, the filling chamber of the filling stage is connectable to a reservoir for hydraulic fluid via a valve which closes when the brake force booster is functioning and opens when the brake force booster is not functioning. Between the filling chamber and the primary chamber (i.e. the high-pressure chamber in the master brake cylinder for supplying the primary circuit of a dual circuit brake system) a sealing sleeve is disposed which allows hydraulic fluid from the filling chamber into the high-pressure chamber but blocks said fluid in the reverse direction.
The invention is based on the problem in a hydraulic brake system of the type according to the preamble of further increasing the functional reliability.